1. Field of the Invention
The present invention relates to an antireflection film provided on the surface of a base material, and more particularly, to an antireflection film and an optical device having excellent antireflection characteristics against a light beam having a wide wavelength range at a wide range of an incident angle.
2. Description of the Related Art
Antireflection films are provided on the surfaces of the base materials for an optical device such as a lens, a prism or the like constituting optical equipments to improve a transmittance of a light. Most of conventional optical equipments have been equipment suitable to a visible range light beam incident at a particular narrow range of an incident angle. Thus, the antireflection films provided on the optical devices have been designed to perform an excellent antireflection effect at the particular narrow range of an incident angle.
However, optical equipments applicable to a light beam incident at a wide range of an incident angle are increasing in the recent years. Thus, there is a demand for an excellent antireflection effect against a light beam incident at a wide range of an incident angle over the whole visible range. Lenses used in optical equipments for image-pickup such as a camera also tends to be a higher numerical aperture in response to a demand for size reduction and higher performance. For example, when the numerical aperture of an objective lens of a camera increases, curvature of the lens increases also, and the incident angle of a light beam on a lens peripheral portion also increases. As a result, ghost or the like is easily generated due to reflection at the lens peripheral portion. Thus, there is also a demand for an excellent antireflection effect against a light beam incident at a wider range of an incident angle over the whole visible range.
For example, Japanese Patent Laid-Open No. 2006-215542 discloses an antireflection film composed of three layers laminated, a dense layer containing alumina as a main component, a dense layer having a refractive index of 1.33 to 1.50 and a porous layer composed of an aggregated nanoparticles of mesoporous silica and having a refractive index of 1.07 to 1.18 in this order from a base material side. The antireflection film disclosed in Japanese Patent Laid-Open No. 2006-215542 performs excellent antireflection characteristics over a wavelength range of 400 nm to 700 nm, the wavelength range of visible light.
Japanese Patent Laid-Open No. 2010-38948 also discloses an antireflection film having a two-layer structure composed of a dense layer and porous layer of a silica aerogel in this order from a base material side. In the antireflection film disclosed in Japanese Patent Laid-Open No. 2010-38948, a refractive index is gradually reduced from the base material to porous layer of the silica aerogel. As a result, it is demonstrated that the antireflection film is excellent in antireflection characteristics against a light beam having a wide wavelength range at a wide range of an incident angle.
However, even the antireflection film disclosed in Japanese Patent Laid-Open No. 2006-215542 achieves a reflectance of 1% or less against a light beam having a wavelength range of 400 nm to 700 nm when the light beam has an incident angle of 30°, further low reflectance is required. In a lens or the like used in recent optical equipments, the incident angle of a light beam sometimes exceeds 30°, and the light beam has a wider wavelength range. Thus, such antireflection film cannot achieve a sufficient antireflection effect against a light beam having a wider wavelength range at a wider range of an incident angle, and insufficient effect on reduction of a ghost or the like may be supposed. Moreover, the shape or the property of the base material may be affected by heat because a firing process at 300° C. or more is required to form the porous layer of mesoporous nanosilica.
In the antireflection film disclosed in Japanese Patent Laid-Open No. 2010-38948, porous layer of the silica aerogel is provided as a surface layer. When porous layer of the silica aerogel is formed to have a refractive index of 1.15 on the surface of a SiO2 layer as the dense layer, drawback that durability in a practical use cannot be achieved due to the inherent property of silica aerogel may occur. In addition, the silica aerogel changes its structure by water adsorption. When the silica aerogel is subjected to hydrophobic treatment using fluoride to prevent water adsorption, the porous layer of the silica aerogel increases a refractive index. Then, for example, it has been reported that when particles of silica aerogel are bonded together by using a binder in the process for formation of porous layer of the silica aerogel to achieve durability in a practical use, a refractive index of the porous layer of the silica aerogel comes to about 1.25 (see “35th Optical Symposium Proceedings,” P67-P70, July 2010, sponsored by the Optical Society of Japan (an affiliate of the Japan Society of Applied Physics), for example).
So, an object of the present invention is to provide an antireflection film and an optical device excellent in antireflection characteristics against a light beam having a wide wavelength range at a wide range of an incident angle, and also having sufficient durability in a practical use.